Relationship Between Sequential Structure, Crystallization, Morphology, Mechanical And Degradation Properties Of Bioderadable Aromatic/Aliphatic Copolyesters

In this dissertation, the relationship between sequential structure and crystallization, morphology, mechanical, degradation properties of biodegradable aromatic/aliphatic copolyesters was investigated, and the effect of transesterification on sequential structure, miscibility and properties was evaluated. The main results and contents are summarized as follows:The sequential structure, crystallization behavior, tensile property and degradation of poly(ethylene terephthalate-co-ethylene oxide-co-lactide) (ETOLA) copolyester was investigated based on melt transesterification of poly(ethylene terephthalate) with poly(ethylene oxide) and oligo(lactic acid). The degree of randomness is calculated to be 0.38, showing of the incorporation of ethylene oxide (EO) block into the homogeneous sequences of ethylene terephthalate (ET) and lactide (LA) units. The analysis of isothermal crystallization kinetics revealed that the crystallization activation energy of copolyester calculated by Arrhenius'equation was lower than that of reported poly(ethylene terephthalate) (PET), indicating that the addition of EO and LA units into PET retarded the crystallization of PET. The copolyester exhibited the same crystal structure at different crystallization temperatures similar to that of PET homopolymer in terms of diffraction peaks, showing of cocrystallizatin behavior. The multiple melting behavior of the copolyester was attributed to the melt recrystallization mechanism. The size of the spherulites of ETOLA increased with crystallization temperature. The increase of crystallization temperature reduced the elongation at break of the copolyesters, as well as the enzymatic degradation rate due to the formation of more perfect and larger spherulites in size.A series of copolyesters were prepared by the direct melt transesterification between poly(trimethylene terephthalate) (PTT) and poly(butylene succinate) (PBS). The sequential structure of the copolyesters was analyzed by the'H-NMR spectra, and the randomness of the copolyesters was calculated to be about 0.8. Cocrystallization, thermal behavior and spherulitic morphology were investigated. Melting points of the copolyesters show a pseudo-eutectic behavior exhibiting the isodimorphic cocrystallization. Wide-angle X-ray diffractogram indicate that the copolyesters crystallized in PTT crystals as the butylene succinate (BS) unit content is less than 60% and in the PBS crystals as the BS unit content is higher than 70%. The mechanical properties of the copolyesters were greatly influenced by the sequence length of the aromatic and aliphatic units. The incorporation of BS units into the PTT structure led to the faster degradation rate of the copolyesters due to the decrease of aromatic sequence length and increase of aliphatic sequence length.The evolution of structure and morphology in poly(trimethylene terephthalate)/poly(butylene succinate) (PTT/PBS) blends induced by the transesterification at different blending temperatures and times was investigated. By control of the extent of transesterification, the degree of randomness, crystallization, morphology and tensile properties of the blends could be modulated. The results indicated that the degree of randomness of the blends increased by increasing of the blending temperature above 260℃and blending time, leading to the formation of miscible blends or even copolyesters. The crystallization of the blends was restricted by the increase of degree of randomness, showing of reduced crystallization and melting temperature, accompanied by the appearance of broad reflection peaks of PTT as determined by X-ray analysis. The morphology of the blend at melting or crystalline state was also determined by the degree of randomness, changing from immiscible to miscible blends as observed by polarized optical miscroscopy (POM). The spherulites became imperfect and showed of small diameters with the increase of blending time and temperatures. The elongation at break increased by increasing of the blending time and temperature, accompanied by the decrease of tensile strength and elastic modulus, showing of dependence on the miscibility and crystallinity in blends, which is determined by the degree of randomness causing by transesterification.